Unsaturated hydrocarbons, such as ethylene, propylene, butylene, isobutylene, and the like, are intermediates that are useful in the petrochemical industry. In order to satisfy market demand, substantial efforts have been invested in the production of such compounds by conventional thermal steam cracking of alkanes and naphtha and by catalytic dehydrogenation methods. However, conventional steam cracking is equilibrium limited and requires very high temperatures (which may exceed 700° C. for the conversion of light hydrocarbons) to achieve a high enough conversion to be economically viable. Even so, temperatures are limited by available alloys to temperatures at which single pass yields are still relatively low. Steam cracking also requires the input of large amounts of heat to drive the gas reaction which is endothermic. Because of the equilibrium limitations, steam cracking is often carried out at low pressures, typically one atmosphere or less, and usually requires cooling and compression of the product stream to effect separation and recovery of the unsaturated hydrocarbons.
Conventional catalytic dehydrogenation has similar disadvantages, including the need for high reaction temperatures (e.g., 550 to over 700° C. depending on the feedstock), the deactivation of the catalyst by coke formation, and the consequent need for continuous or periodic catalyst regeneration at frequent intervals throughout the process. In addition, there are thermodynamic limitations in conventional dehydrogenation. These thermodynamic limitations are due to the fact that conversion in conventional dehydrogenation processes are equilibrium limited, and require high temperature and low pressure to achieve high single pass yields. As a result of these substantial drawbacks, the petroleum industry has sought a solution to the demand for olefinic hydrocarbons in the use of autothermal cracking and catalytic oxidative dehydrogenation methods.
In autothermal cracking, oxygen or air is added to the feed and partially combusts part of the feed in situ generating the high temperatures required to thermally crack the remaining feedstock. In some variants a catalyst is used to support combustion with the catalyst being in the form of a fixed bed or a fluidized or spouted bed. In some cases hydrogen is co-fed with the feedstock and is found to increase olefin yields. Autothermal cracking usually takes place at high temperatures (550-1200° C.) and requires very short reaction times and rapid quenching of the products to preserve the olefinic products and prevent further undesirable reactions. Even so, by-products are typically formed including carbon oxides. At higher pressure, yields of undesirable by-products increase. At very high temperatures as encountered in some autothermal processes, hydrocarbon cracking to methane also reduces selectivity to useful olefinic products.
Catalytic oxidative dehydrogenation is, in principle, not subject to many of the problems associated with conventional steam cracking or catalytic dehydrogenation because of the presence of oxygen in the reaction mixture. One route of oxidative dehydrogenation uses oxygen to react with the hydrogen released from the hydrocarbon, in situ, so that the aforementioned equilibrium limitation is removed, and high single pass yields can be achieved. The reaction is exothermic overall and does not require a supply of heat as in endothermic dehydrogenation reactions. Generally, in a catalytic oxidative dehydrogenation process, the reactants (hydrocarbon and an oxygen-containing gas) are passed over a catalyst to produce an olefin product. Typically, the hydrocarbon is a saturated hydrocarbon such as ethane or a mixture of saturated hydrocarbons. The hydrocarbon may be gaseous or liquid at ambient temperature and pressure but is typically gaseous. However, there are no reported commercial oxidative dehydrogenation processes operating at this time and despite extensive research, there remains a need for new oxidative dehydrogenation catalysts, catalytic systems, and methods that achieve high conversion at high selectivity, such that the yield of the desired olefin is maximized, and extraneous oxidative side reactions are minimized.
The disclosed invention provides a solution to these problems. In at least one embodiment of the invention, the conversion of one or more hydrocarbons to one or more unsaturated hydrocarbons may be achieved with relatively high selectivities to the desired product as a result of conducting the process in a microchannel reactor without the use of catalyst.